1. Field of the Invention
This invention relates to apparatus and processes facilitating design of full-scale chemical processes and associated equipment as well as emergency relief systems, and which may be carried out on a micro-scale basis in a manner that not only fully simulates the normal functioning and operating parameters of a full-scale process but also any overpressure conditions which can occur.
2. Description of the Prior Art
Many chemical processes are carried out in pressure vessels even though the process itself may be operated at minimal, atmospheric or subatmospheric pressures. The pressure vessel is designed and sized to accommodate specified overpressure conditions which may occur for a variety of reasons. Desirably, the pressure vessel used has emergency relief structure which will allow pressure which builds up in the vessel to be safely relieved therefrom before rupture of the vessel can occur.
Much effort has been expended in the past to predict the conditions that may occur in a chemical process carried out in a pressure vessel that will require pressure relief and to ascertain the vent structure that should be provided to handle such overpressure before catastrophic failure of the vessel can take place. In certain instances, the vent systems provided have been grossly oversized to avoid any possible problem in vessel rupture. This inherently increased the cost of the equipment but was felt necessary because of the recognition that the design was predicated primarily on estimates of possible overpressure conditions rather than reliable data. In other cases, vent systems have been inadequate to provide effective emergency relief because of inability of the process designers to accurately estimate the conditions that may actually arise under an overpressure situation.
The chemical process industry has also been plagued with problems of unscaling a specific process from laboratory equipment and test setups without the necessity of piloting the process through one or more incremental stages of increasing size. Typically, at least one pilot plant must be built and operated intermediate the lab simulation and a full-scale process. Even in these instances though, the pilot facility may not in all instances be an accurate test bed for what will occur in a full-scale chemical process, particularly from the standpoint of possible dangerous overpressure conditions where processing in pressure vessels is carried out. Furthermore, it has not heretofore been possible in many instances to determine whether or not chemical processes which take place with certain results at specific efficiency rates will operate in essentially the same fashion when carried out on a full-scale basis.
The difficulties of scaling up chemical processes from a procedure as well as equipment standpoint and of accurately determining what will occur under overpressure conditions have heretofore defied effective solution except through overdesigned equipment, or by carrying out tests and evaluations on a somewhat less than full-scale but substantial basis which is expensive, time-consuming and often impossible from a process and equipment standpoint to realistically duplicate.
Efforts to size emergency relief systems or to fully predict the operation of chemical processes on a pilot or test basis as compared with full-scale operation of a process have been particularly vexatious in connection with exothermic chemical processes which take place in closed reactor vessels. A number of conditions can lead to a runaway reaction and uncontrolled self-heating pressure build-up in the reactor vessel. Examples of such conditions include loss of cooling or refrigeration, contanimation of the reactor contents and overfilling with a critical reagent attributable to faulty instrumentation or erroneous control.
Overpressure conditions including those which occur as a result of a runaway exothermic reaction are principally controlled by the provision of an emergency relief system which operates to release the pressure in the vessel to a safe area at a threshold pressure value above the working pressure of the vessel but substantially below its rupture pressure. The rupture pressure often is set at a value approximately one-fourth that of the pressure which would result in catastrophic failure of the vessel. Typically, a relief system has a vent which opens at a predetermined pressure to allow flow of the reactor contents to appropriate treatment facilities which safely deal with the vented material. For example, if the gases or vapors are toxic or harmful, neutralization steps or gas scrubbers are employed to detoxify or neutralize the chemical agents before release to the atmosphere. If flammable vapors are involved, suitable flare systems or incinerators may be designed to prevent release or accomplish combustion or decomposition of the vapor flow.
Prior to the present invention, the traditional design of emergency relief systems and treatment facilities was based upon the assumption that only gases or vapors were required to be vented. This assumption, however, was seldom fulfilled when a runaway reaction caused a relief vent to open. In fact, what occurred was the venting of a frothy mixture of gas and liquid; i.e. a two-phase flow, which in the extreme involved the entire contents of the reactor vessel. As a result, for a given size relief vent, liquid being discharged filled a portion of the vent and effectively reduced the area available for the venting of vapor. Because less vapor than was assumed was vented per unit time, the pressure in the reactor vessel continued to rise potentially above the reactor vessel design limits. A second problem arose in that, because of the two-phase flow, the mass of the material being discharged was substantially greater than assumed for an all vapor flow. This higher mass flow often rendered any treatment facilities ineffective.
In order to establish an appropriate emergency relief system design, it was necessary to determine the adiabatic self-heat rate of the particular chemical reaction at the design relief pressure for the reactor vessel being used. Prior to the present invention, an accelerating rate calorimeter was used in estimating the self-heat rate. That equipment typically involved a test cell having a large heat capacity relative to that of the test sample. Because the test cell had a relatively high thermal mass, extrapolation of the test results to a full size process reactor was difficult, if not impossible, without detailed kinetics data on the chemical involved. Such data was generally not available on the runaway condition of interest in the design of an emergency relief system.
Similar independently variable factors were encountered in the sizing-up of laboratory developed processes to full-scale operations. The only available solution was to pilot the commercial installation, often done in successively greater incremental sizes to minimize the risks associated with design of an ultimately inefficient, unsafe or inoperable full-scale process.